1. Field of the Invention
This invention relates to machine tools and, in particular, to a system for detecting error in the positioning of a machine tool and for stopping machine tool operation before the mispositioning error causes destruction or damage of the part under manufacture.
2. Description of the Prior Art
Machine tools are well known in the art. They are used extensively, for example, by aircraft manufacturers to construct precision frame members used in the air frames of general purpose and military aircraft. Because of their precise construction, individual parts manufactured on the machine tools for which the invention disclosed hereinafter finds application represent a substantial economic investment.
Most computer operated machine tool devices provide some form of servo error detection on an individual axis basis. Commonly utilized servo error detection circuitry utilized in conjunction with those devices use the electronic measuring of a "following error" in order to determine machine positioning malfunction. A malfunction is detected only when the "following error" is larger than some predetermined "threshold error", generally on the order of 0.100 to 0.200 inches or larger. Such a magnitude of positioning malfunction can destroy or severely damage the workpiece being machined unless the machinery operation is immediately ceased.
An improved monitoring system has been developed which checks the magnitude of the "actual following error" value against a "predicted following error" value based on velocity of the slide holding the particular milling tool. While this latter device may detect positioning error an order of magnitude smaller, i.e., 0.010 inches, the system checks only the performance of the servo electronics, and does not take into account errors in the input command decoding logic. Additionally, both of the above systems depend on the tool's existing feedback system in order to perform their checks. Consequently, a number of malfunctions still may occur resulting in loss of machine time and in loss of material.